Photography using silver halide has been most widely used since the photographic characteristics of silver halide systems such as sensitivity, gradation control, etc., are superior to those of other photographic processes such as electrophotography or a diazo process. Recently, an improved photographic technique has been developed capable of simply and rapidly forming an image, where the image formation of a silver halide light-sensitive material is carried out by means of a dry process under heat instead of a conventional wet process using a developing agent or the like. Heat developable light-sensitive materials are known in this technical field, and various heat developable light-sensitive materials and processes are described, for example, in Shashin Kogaku no Kiso, pp. 553-555 (Corona Publishing, 1979); Eizo Joho (April, 1978), p. 40; Neblett's Handbook of Photography and Reprography, pp. 32-33, 7th Ed. (Van Nostrand Reinhold Company); U.S. Pat. No. 3,152,904, 3,301,678, 3,392,020, 3,457,075, British Pat. No. 1,131,108, 1,167,777, and Research Disclosure (RD-17029), pp. 9-15 (June, 1978).
Many processes for formation of color images are known. In a method for formation of color images by the reaction of an oxidized developing agent and a coupler, U.S. Pat. No. 3,531,286 has proposed a combination of a p-phenylenediamine reducing agent and a phenolic or active methylene coupler; U.S. Pat. No. 3,761,270 describes a p-aminophenol reducing agent; Belgian Pat. No. 802,519 and Research Disclosure, pp. 31-32 (September, 1975) describe a sulfonamidophenol reducing agent; and U.S. Pat. No. 4,021,240 has proposed a combination of a sulfonamidophenol reducing agent and a tetra-equivalent coupler.
For formation of positive color images by a light-sensitive silver dye bleaching process, Research Disclosure (RD-14433), pp. 30-32 (April, 1976) and (RD-15227), pp. 14-15 (December, 1976) and U.S. Pat. No. 4,235,957 disclose useful dyes and a bleaching process.
In addition, European Pat. Nos. 76,492 and 79,056 and Japanese Patent Application (OPI) Nos. 28928/83 and 26008/83 disclose a method for formation of color images by heat development using a compound initially having a dye moiety and capable of releasing a diffusible dye corresponding to or reversely corresponding to a reduction reaction in which a silver halide is reduced to silver at a high temperature (the term "OPI" as used herein refers to a "published unexamined Japanese patent application").
Such heat developable light-sensitive materials are developed under heat in a substantially water-free state. Since the heat development of the photographic material is a dry treatment, simple and rapid formation of an image is possible, which is an advantage of these heat developable light-sensitive materials.
Despite this significant advantage, heat developable materials have some troublesome problems. For example, since it is not possible to supply any photographically useful reagents from a developer or other solution, all of the necessary reagents have to be incorporated in the light-sensitive material. Accordingly, when a certain photographically useful reagent is to be formed at the beginning of development, or is to be formed, released or diffused with a desired timing, selection of only the desired reaction or cooperation of two or more reactions is technically most important. In particular, a technique for formation, release and diffusion of a development restrainer with a certain timing after development begins, for the purpose of inhibition of fogging due to hyper development, development termination after a certain period of time and inhibition of image density mottle due to uneven heating, is extremely important, and will determine the image quality of the heat developable light-sensitive materials.
Certain timing techniques for releasing a photographically useful reagent during the photographic treatment of a photographic material have heretofore been known in conventional and instant photography systems, including the use of a polymer shielded layer which becomes permeable with a desired timing or the use of a reagent having an active part that is previously blocked with a protective group which may be de-protected during development. However, these conventional means cannot be utilized in a heat development system, or if used, provide an insufficient effect in this system. Under the circumstances, a novel and effective timing technique which is efficiently applicable to the heat development system is required.